A communication technology which enabled global diffusion of mobile phones has been used for conventional communication/broadcasting, and, in addition, the technology has been diligently researched and developed by relevant organizations to achieve a wireless network mainly intended to monitor/control a social infrastructure apparatus, which is required to perform highly reliable and highly secure communication.
In a controlling/monitoring network of social infrastructure apparatuses, in order to limit a communication service area within an area of an infrastructure system and in order not to interrupt operation of apparatuses constituting the infrastructure system, it is desired to constitute a mesh network in which wireless devices placed in the respective apparatuses communicate with each other.
In the mesh network, it is difficult to have a remarkable difference in height between a transmitter station and a receiving station. Further, because electromagnetic waves emitted from the wireless devices are scattered by the apparatuses, communication is performed by using a multi-reflected wave which is a non-line-of-sight wave. The non-line-of-sight wave is received at a field strength which is generally lower than that of a line-of-sight wave. In communication using a plurality of non-line-of-sight waves, there is a possibility that a plurality of reflected-wave propagation paths having substantially the same propagation attenuation characteristic are formed between a transmission side and a reception side and distinctive communication is achieved by using those reflected-wave propagation paths.
An electromagnetic wave has a characteristic that receives rotation of an inherent polarization vector when the electromagnetic wave is reflected by a scatterer in relation to both a direction of a normal vector to a surface of the scatterer and a direction of a polarization vector entering the scatterer. In view of this characteristic, a receiving station can know a polarization direction of an electromagnetic wave emitted by a transmitter station and can also know a polarization direction of an electromagnetic wave received via a plurality of reflection propagation paths reaching this receiving station. It is possible to achieve special communication in which the plurality of propagation paths are recognized or selected on the basis of both the directions.
In order to achieve the above communication, it is necessary to change a polarization direction of an electromagnetic wave and achieve a device for detecting this polarization direction. As an electromagnetic wave whose polarization is rotated, a circularly polarized wave is known. In the circularly polarized wave, a rotation frequency of the polarized wave and a propagation frequency are the same. Generally, a frequency range of an electromagnetic wave for performing wireless communication using a non-line-of-sight wave is limited to several hundred MHz to several GHz. Thus, the rotation frequency of the circularly polarized wave also falls within a range of several hundred MHz to several GHz, and therefore an oversampling ratio of 4 to 8 or more for performing accurate digital signal processing cannot be obtained at several hundred MHz which is an operation frequency of present commercial digital signal processing devices. By using an electromagnetic wave in which a rotation frequency of a polarized wave is lower than a propagation frequency, a polarization angle of an electromagnetic wave having a frequency for performing favorable communication with the use of a non-line-of-sight wave can be controlled or detected in a commercial digital signal processing device. The above electromagnetic wave is referred to as a rotationally polarized electromagnetic wave, and, by using, for example, two electromagnetic waves having different frequencies, it is possible to form a special electromagnetic wave in which a rotation frequency of a polarized wave is a half of a difference between both the frequencies and a propagation frequency is a half of the sum of both the frequencies. The above special electromagnetic wave can be achieved by composing two circularly polarized waves having different frequencies and different rotational directions, and therefore it is required to achieve an antenna which simultaneously generates those two circularly polarized waves.
Regarding such a request, there is known a configuration in which two antennas which generate electromagnetic waves having different frequencies and different rotational directions are achieved by a microstrip antenna having a thickness and are stacked.
In Purpose of Abstract of PTL 1, there is described “to provide a shared microstrip antenna for two frequencies, which includes a circularly polarized patch antenna for transmission and a circularly polarized patch antenna for reception, has large isolation between a transmitting terminal and a receiving terminal, and has a simple feeder circuit configuration”.